Recently, as mobile phones include a camera function, image data captured by cameras is transferred to devices such as television sets and printers, and the devices perform a predetermined process such as an image display process.
Examples of an interface between mobile phones and television sets, printers, and personal computers are infrared methods such as IrDA (Infrared Data Association) (see Non-Patent Documents 1 and 2).
Since the infrared ray used for infrared communication such as IrDA has directivity, a shield between the devices will completely disable data transfer. However, if there is no shield between the devices, high-speed data transfer is possible. IrDA is broken into Very Fast IR (VFIR) with a maximum speed of 16 Mbps, Fast IR (FIR) with a maximum speed of 4 Mbps, and SIR with a maximum speed of 115.2 kbps. However, in commercially available products, the maximum speed is up to 4 Mbps.
FIG. 14 shows a procedure of establishment of connection of an IrLAP layer through the IrDA standard, which is one of standards of infrared communication.
A primary station serves to look for the other station at the beginning of transmission. That is, the primary station requires establishment of data transfer state and outputs a “station search command (XID command)”. On the other hand, a secondary station serves to accept the request and outputs a “station search response (XID response)”. A request (order) from the primary station to the secondary station is called a command, and a reply from the secondary station to the primary station is called a response.
The XID command serves to look for a potential secondary station within a certain distance in which the primary station is capable of transmission. The slot numbers indicative of numbers in parentheses denote ascending numbers sequentially put on the commands.
After receiving the XID command, the secondary station outputs the XID response, which is a station search response, so as to notify the primary station of its existence. The primary station outputs a predetermined number of XID commands, and then outputs the final XID command labeled with the slot number 255. That is, the command with the slot number 255 is the final command (XID-END).
Next, the primary station notifies the secondary station of parameters required for communication, such as maximum transferable speed and maximum receivable data length, using a SNRM command. The secondary station receives the command and carries out comparison between the received setting value and its own setting value, and then transmits a UA response to the primary station so as to notify an allowable value.
The following more specifically describes establishment of data transmission state. In IrDA standard, the number of packets of XID command transmitted from the primary station is selected from 1, 6, 8, and 16. The number is described in a Discovery flag of the XID command. As shown in FIG. 14, when 8 packets of XID command are transmitted each time, the 1st to 8th packets are given slot numbers from 0 to 7. Lastly, the XID-End command with the slot number of 255 is transmitted so that the receiving end will notify that it is the final packet. Then, after approximately 500 m seconds since the final packet is sent, the 1st through 8th packets are transmitted again. These packets are transmitted with a time interval of 25-85 m seconds.
The secondary station does not always output the XID response immediately after receiving the XID command, but outputs the response after receiving a packet with an arbitrary (a random value) slot number. For example, when 8 packets of XID command are transmitted each time, the secondary station arbitrarily determines whether it outputs the XID response after receiving the 1st packet or after receiving the 8th packet. For example, in FIG. 14, the secondary station outputs the XID response after receiving the third packet.
According to SIR, IrDA standard specifies that the XID command and the XID response are transferred at 9600 bps. This is much slower than the transfer speed of data frame (described later), which is sent at 4 Mbps. Consequently, transmission/reception of the XID command and the XID response takes a longer time. Through these processes, a procedure for searching stations between the primary station and the secondary station is completed.
After the procedure, the primary station notifies the secondary station of parameters required for communication, such as maximum transferable speed and maximum receivable data length of the primary station, using a SNRM command. The secondary station receives the command and notifies the primary station of parameters required for communication, such as maximum transferable speed and maximum receivable data length of the secondary station, using a UA response, thereby establishing an IrLAP connection between the primary station and the secondary station, in which connection maximum transferable speed, maximum transferable data length, and other parameters are set.
After the IrLAP connection using an IrLAP command packet, IrLMP connection, TinyTP connection, and OBEX connection are established using IrLAP data packets.
A request for an IrLMP connection from the primary station is transferred to the secondary station using an IrLAP data packet and a response for an IrLMP connection from the secondary station is transferred to the primary station using an IrLAP data packet, thereby establishing an IrLMP connection.
Then, a request for a TinyTP connection from the primary station is transferred to the secondary station using an IrLAP data packet and a response for a TinyTP connection from the secondary station is transferred to the primary station using an IrLAP data packet, thereby establishing a TinyTP connection.
Further, a request for an OBEX connection from the primary station is transferred to the secondary station using an IrLAP data packet and a response for an OBEX connection from the secondary station is transferred to the primary station using an IrLAP data packet, thereby establishing an OBEX connection. As a result, connections allowing for information data transfer are established.
For disconnection, a request and a response for OBEX disconnection, a request and a response for TinyTP disconnection, and a request and a response for IrLMP disconnection are transferred using IrLAP data packets, and then a DISC command which is a request command packet for IrLAP disconnection and a UA response which is a response command packet are transmitted, thereby disconnecting a communication between the primary station and the secondary station.
Further, in IrDA, communication is performed on a frame basis. FIG. 16 shows a frame in IrDA, which is constituted of a preamble field, a start flag, an address field, a control field, a data field, a FCS, and a stop flag. Among these, the preamble field is used for generation of reception clock which is used in a reception circuit of the receiving end. Further, the FCS includes an error detection code for detecting an error.
Further, there are various kinds of frames, such as an I (Information) frame for information transfer, an S (Supervisory) frame for supervision/control of communication, and a U (Unnumbered) frame for connection or disconnection of communication. The information for identifying those I, S U frames is contained in the control field.
Since data transmission cannot be completed within one frame in most cases, the data is divided into plural I frames or plural UI frames. Each of I frames contains data to be transmitted in the data field, and is given a sequence number by which any omission of data can be found. With this arrangement, highly-reliable communication is realized. Each of UI frames contains data to be transmitted in an I field, but is not given a sequence number by which any omission of data can be found. The S frame has no data field for containing data, and is used for transmission of notification of condition, such as establishment of communication, or a busy state, or used for a request for retransmission or the like. The U flame is called an unnumbered frame as it has no sequence number like those of the I frames. The U frame is used for setting of communication mode, report of response and irregular condition, or establishment of data link.
[Non-Patent Document 1] Infrared Data Association Serial Infrared Link Access Protocol (IrLAP) Version 1.1 (Jun. 16, 1996)
[Non-Patent Document 2] Infrared Data Association Serial Infrared Physical Layer Specification Version 1.4 (May 30, 2003)
However, the above arrangement is problematic in that data transfer takes much time and efficiency for data transfer decreases.
That is, as described above, for establishment of communication, IrDA requires sensing of 500 ms, searching for stations using an XID command, exchange of negotiation parameters using SNRM and UA.
When plural communication stations exist in a communication area, it is necessary to control stations in the area so that a communication is not prevented by light emitted from an unexpected station. However, an infrared ray has high directivity and infrared ray communications are mostly one-to-one communications. Therefore, it is not always necessary for a device to automatically search for other stations in a communication area.
Further, IrDA standard specifies that a search for other stations using XID is performed at 9600 bps, which is comparatively lower than 4 Mbps at which data exchange is performed. Further, as shown in FIG. 15, for information data transfer, after completing connection of IrLAP, an IrLMP layer, a TinyTP layer, and an OBEX layer are serially connected by serially exchanging IrLAP data packets. For disconnection, OBEX, TinyTP, IrLMP, and IrLAP are disconnected in the same order.
For the above reason, in a case where one information terminal transfers data to another information terminal using an infrared ray, whole transfer efficiency drops due to a time for preparing data transfer and a time for disconnection.
Further, in a case where a portable terminal transfers an image to an image display device, when a transmission error occurs, a user sometimes can be aware the transmission error with ease. For example, a case where an image display device at a receiving end displays an image different from an image the user has transferred, or a case where the image display device does not renew its display.
If it takes little time for a user to see whether user's transmission of data is performed successfully or not, it is not so troublesome for the user to retransmit the data even if an assured and errorless communication is not performed. Such a device only requires an ability to receive data. A transmission circuit and a transmission device may be omitted from the device.
However, since IrDA standard specifies that a receiver outputs a response to a command, all receivers must include a transmission circuit and a transmission device.